As others have said Google Voice, Google Maps/Navigator, Wifi and USB tethering.

SIPDroid for free VOIP calls from anywhere and a US phone number when overseas. But don't use the version in Market because it is disabled on 3G and EDGE to appease cellular operators. Instead download the apk from http://www.sipdroid.com/ and install it from your SD card.

And of course to install from your SD card, a good file manager would be helpful. As others have said, ASTRO File Manager, free from Market, is a must have.

Zillow's app, free from Market, is awesome for home buyers for its cool integration with Google Maps.

Mapdroyd (with a y), free from Market, is no competition for Google maps but is a great backup because it can download it's maps in advance and then works great off-line when you land is some third world country and are taking a taxi to your hotel with no data access.

Speed Test, free from Market, is a fun little app for checking up on your data rate performance under different circumstances.

And not qualifying as app's but pretty awesome for the more tech savvy types with rooted phones and voided warranties . Amon-RA's custom recovery from http://forum.xda-developers.com/showthread.php?t=611829 is a must have for changing ROM's and tinkering with other stuff you aren't supposed to tinker with.

And last but not least for the more tech savvy types with rooted phones and voided warranties CyanogenMod's custom ROM's from http://www.cyanogenmod.com/ are generally really awesome and while their are many custom ROM's out there, CyanogenMod's are generally pretty polished and release quality. Have an old G1 and want most of the same new features of a shiny new Nexus One running Android 2.1 on your old beater? Try the latest CyanogenMod ROM.